Anal. Chem. 1981, 53, 1390-1394
1390
Derivative Linear Sweep and Derivative Cyclic Volta bsorptometry Eric E. Bancroft, James S. Sidwell, and Henry N. Blount' Brown Chemical Laboratoty, University of Delaware, Newark, Delaware 1971 1
A new spectroeiectrochemicai technique has been developed which couples linear and cyclic linear potential sweep perturbation of an optically transparent electrode (OTE) with optlcal monitoring of the light-absorbing product of the electrode reaction. For the 0 t ne- = R couple, the absorbance of the product of the electrode reaction ( A R ) Is differentiated with respect to the linearly varying potential (dAR/dE) and is displayed as a function of sweep potential. The techniques are termed derivative linear sweep voltabsorptometry (DLSVA) and derivative cyclic voltabsorptometry (DCVA). The dAR/dE vs. E waveforms are morphologically identical with the ideal linear sweep or cyclic voltammetric responses for the electrochemical process. The analytically significant derivative optical peak amplitude is given by (dA,/dE)p = pnl/2,RI)01/2C v-1/2 where p = -0.0881 mV-1'2. This functional form has been experimentally verified by using four well-characterized electrochemical systems. For the twoelectron oxidation of o-tolidine at a gold OTE in pH 2.00 phosphate buffer, the limit of detection of this technique has been found to be 3.8 X lo-' M at a scan rate of 2.5 mV/s.
Since its inception in the mid-60s (I), the technique of absorption spectroelectrochemistry has been widely utilized to study the redox chemistry of numerous systems. This methodology which combines electrochemical perturbation of an optically transparent electrode (OTE) with simultaneous absorption spectrophotometric monitoring of the solution adjacent to the electrode surface has been successfullyapplied to the delineation of kinetic parameters characteristic of homogeneous chemical reactions which are coupled to the primary electrode process as well as to the evaluation of thermodynamic system parameters (2-6). Indirect coulometric titrations with optical monitoring have provided significant data on formal potentials and n values of biological redox components (4, 5 ) . Potential step absorption spectroelectrochemistry (chronoabsorptometry) has also proven to be a viable tool for the determination of heterogeneous electron transfer kinetic parameters at both normal (6) and chemically modified (7, 8) OTEs. Apart from indirect coulometric titrations ( 4 , 5 ) ,absorption spectroelectrochemistry has enjoyed virtually no analytical applications. This is principally due to the morphology of the absorbance-time transient which is observed when potential step excitation techniques are employed. For the electrode process represented by
0 + ne- = R
Q ( t )= 2nFADo1f2Co*t1/2/~1~2
(3)
where F is the Faraday constant and A is the electrode area. Both AR(A,t) and Q(t) represent the integral of the flux of eq 1. It follows, then, that the derivative of the absorbance with respect to time is analogous to that of the charge with respect to time, namely, the current. Thus d&(A,t)/dt provides an alternative measure of the flux of the process of interest and does so with both the molecular specificity attendant to the use of optical monitoring and insensitivity to nonfaradaic charge-consuming processes (11). Moreover, the absorbance derivative suggests numerous analogues of electroanalytical techniques which can be expected to offer many advantages for the analysis of low concentrations of materials, the analysis of mixtures, the diagnosis of mechanisms of coupled chemical reactions as well as the evaluation of kinetic parameters which are characteristic of those reactions, and the determination of heterogeneous electron transfer kinetic parameters. This report describes the optical analogue of linear sweep and cyclic voltammetry in which the absorbance of the product of the electrode reaction (AR, eq 1) is differentiated with respect to the linearly varying scan potential and is displayed as a function of the scan potential. The resulting dAR/dE vs. E waveforms are morphologically identical with their ideal current-potential analogues (Figure 1)and the methods are termed derivative linear sweep uoltubsorptometry (DLSVA) and derivative cyclic voltubsorptometry (DCVA).
THEORY For the electrode process given by eq 1,the faradaic current observed under conditions of linear potential scan ( E = Eiu t ) may be expressed as (12, 13)
i = n F A D o ~ f 2 C o * u i f 2 E 1 / 2 ~ 1 ~ 2 x ( x ) (4) where v is the rate of potential sweep, F = nF/RT, and the transcendental function d Z x ( x ) (also known as the current function where x = nF(Elj2- E ) / R l ' )may be represented in series form as (14) m
n1f2x(x)= -E(-1)jj1/2 expGtut - j t u )
(5)
j=l
where AE = Ei - El,*, Ei is the initial potential, and E l l z is the polarographic half-wave potential. Substitution of eq 5 into eq 4 and subsequent integration afford the charge observed under conditions of linear potential scan, namely m
(1) application of a potential step of sufficient amplitude to cause reaction 1to proceed a t a diffusion-controlled rate results in the time-dependent absorbance of R given by (9) AR(X,t) = 2t~(X)Do'/'Co*t'/~/rr'/~
charge passed under the same experimental conditions, namely (10)
(2)
where CR (A) is the molar absorptivity of R, Do is the diffusion coefficient of 0, and CO*is the bulk concentration of 0. This functional form is analogous to that noted for the faradaic
Q = -nFADo1/2Co*E-1f2u-1/2~ (-l)'j-'/'[expGtvt
-
j=1
i t a ) - expO'taE)I (6) The absorbance of R is related to the charge passed in producing R according to AR = 103cRQ/nFA
(7)
Substitution of eq 6 into eq 7 and subsequent differentiation with respect to the electrode potential, E, and rearrangement
0003-2700/81/0353-1390$01,25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981 I
730
I
I
520
31 0
-300
100
E (mV vs SCE) Flgure 1. Simultaneously acquired current-potential (A) and absorbance-potential (B) responses for the cyclic linear sweep oxidation of 3.16 mM TAA at a platinum OTE in acetonitrile/O.100 M TEAP at Y = 551 mV/s. Trace (C) is the derivative of trace (B) with respect to scan potential. AU = absorbance unit; arrows indicate initial scan direction.
gives rise to the functional form of the derivative optical signal shown in eq 8. Although the series representation of the
dAR/dE = m
CRD~~I~C~*V-'I~F~I~C (-l)jj1J2 expG[vt
- j4AE)
j=1
(8)
current function given in eq 5 is mathematically convenient for the manipulations described above, it is not well behaved for x > 0 which includes the analytically significant region of the voltammetric peak. Oldham (15) has recently reported several alternative series representations of the current function which are well behaved for all values of x . One particularly useful form is (15) m
T'/~x(x= ) L
1391
+ MX- Nx' + ( T / ~ k)="l (~y -Cx)"'(Y + 2x)/y3 - (8b2 + 12bx - 15x2)/8b7/' (9)
where L = 0.380104813, M = 0.118680871, N = 0.043920560, y = [(Zk - 1)2n2+ x2]1/2, and b = (2k - 1)n. At the voltammetric peak (x = 1.1090), T ' / ~ X ( X ) is numerically equal to 0.44629 a t 25 "C (15, 16). Substitution of this value into eq 8 gives rise t o the optical analogue of the Randles-Sevcik equation, namely (dAR/dE), = /3n'/2~RDo'/2Co*~-'/2
(10)
where /3 = -0.088 10 mV-1/2,the concentration is in mol/L, and the scan rate is in mV/s.
EXPERIMENTAL SECTION Materials. Potassium ferrocyanide (Mallinckrodt, Analytical Reagent) was used as received. Solutions of this reagent were prepared gravimetrically in pH 2.50 glycine buffer (0.50 M). Methyl viologen (Aldrich) was recrystallized three times from absolute ethanol and vacuum dried prior to preparation of sample solutions in pH 7.00 phosphate buffer (Titrisol,Merck) containing 0.100 M sodium chloride (Fisher). o-Tolidine (Fisher) was recrystallized three times from absolute methanol and vacuum dried (mp 130.5-131.5 "C, lit. (17) 130 "C). Solutions of this reagent were prepared in pH 2.00 phosphate buffer (0.50 M). Tri-p-
-500 -700 E ( r n V v s SCE)
-900
Flgure 2. Variation of DCVA response with scan rate for 1.55 mM MV2+ at tin oxide OTE in pH 7.00 phosphate buffer. Scan rates are as follows: A, 25.0 mV/s; B, 50.0 mV/s; C, 97.2 mV/s; D, 265 mV/S.
anisylamine was prepared after thc.manner of Wieland and Wecker (18). Sources of and purification procedures for acetonitrile and tetraethylammonium perchlorate (TEAP) have been reported elsewhere (19,20). Deionized, glass distilled water was used in the preparation of all aqueous solutions. All sample solutions were prepared immediately prior to use and were deoxygenated with prepurified nitrogen (Linde) which had been passed over hot copper turnings and presaturated with the appropriate solvent. Concentrations of methyl viologen, o-tolidine, and tri-p-anisylamine were determined spectrophotometrically. Apparatus. All electrode potentials are reported relative to the aqueous saturated calomel electrode (SCE)and were controlled by a potentiostat of conventional design (6). Fluoride-doped tin oxide (PPG Industries, Pittsburgh, PA),vapor deposited gold @ I ) , and vapor deposited platinum (21)O m s were fitted to cells which are similar in design to those already reported (22, 23). Both the potentiostat and the single-beam spectroelectrochemical apparatus (24) were interfaced to a dedicated NOVA 1200 (Data General Corp.) computer system (25) for both the simultaneous acquisition of electrochemical and optical signals and subsequent data processing. All measurements were made at 25.0 (A0.6)"C. Computational Algorithms. Following data acquisition, the potential-dependent optical intensity was converted to an absorbance-potential file. The absorbance elements of this array were then subjected to two passes of a five-point moving average smoothing routine (26) followed by differentiation of the optical absorbance (26)with respect to potential. The resulting dA/dE vs. E waveform was then displayed as shown in Figures 2 and 3.
RESULTS AND DISCUSSION To demonstrate that the dA/dEvs. E and i vs. E waveforms are morphologically identical, absorbance and current data were simultaneously acquired as a function of linearly varying potential for the monoelectronic oxidation of tri-p-anisylamine (TAA) to its corresponding cation radical (TAA'.) and subsequent reduction of T U + .back to TAA a t a platinum OTE in acetonitrile containing 0.100 M TEAP. Shown in Figure 1 are the observed current-potential and cation radical absorbance-potential responses together with the derivative optical (dAT,+./dE vs. E ) waveform. The cyclic voltammetric (CV) and derivative optical (DCVA) responses are in excellent agreement.
1382
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
~~
Table I. Properties of Electrochemical Systems of Interest to This Work c ( A *,y1 1
V? cm * precursor
lo6
medium
4.84 aqueous phosphate (0.06 M ) buffer, pH 7.00, 0.10 M NaCl Fe(CN),4- aqueous glycine (0.50 M ) 6.55 buffer, pFI 2.50 TAA acetonitrile, 0.10 M TEAP 10.1 OT aqueous phosphate (0.50 M) 5.62 buffer, pH 2.00
NV2+
OTE
electrode reaction
tin oxide
MV"
tin oxide platinum gold
Mcm-'
species Amax, monitored nm
s-1 x
+ e - MV'.
X
MV'.
605
1.25'
Fe(CN),4- = Fe(CN)63-+ e -
Fe(CN),3-
420
0.102'
TAA = TAA'. + e O T = OT2++ 2 e -
TAA +. OT2+
715 437
3.30d 6.20'
Determined from single potential step spectroelectrochemical responses ( 9 ) and known molar absorptivities (see above). Refertnce 27. Reference 9. Reference 2. ' Reference 28, 29.
a
I
I
I
Table 11. Dependence of (dA/dL:), on Scan Rate for the Reduction of MV2+a , c
:r\
T
v,
mV/s
u-1/2 ( d A / d E ,) , ( r n V / ~ ) ' ' / ~AU/mV x l o 5
2.50 0.632 235. 4.94 0.450 169. 15.0 97.2 0.258 25.0 75.0 0.200 50.0 54.2 0.141 97.2 0.101 36.9 158. 0.0796 28.4 265. 0.0614 22.1 M ; CMV+'L)'''MVZt = 27.5 M-' a [MV2+] = 1.55 X s at 605 nm; solution conditions given in Table I. Regression statisMean of triplicate determinations. tics for plot of (dA/dE), vs. u - ' / ~ :slope = 3.74 (t0.02) X l o - ? ; intercept = -0.15 (20.67) X 10 5 ; coefficient of correlation = 0.999 90.
'
--
800
600
400
200
E (mV vs SCE) Flgure 3. Variation of UCVA response with scan rate for 9.23 X M OT at gold OTE in pH 2.00 phosphate buffer. Scan rates are as follows: A, 2.72 mV/s; B, 4.87 mV/s; C, 8.99 mV/s; D, 15.5 mV/s; E, 27.1 mV/s; F, 48.6 mVJs; G, 90.6 mVIs.
Examination of eq 10 reveals two unique features of the functional form of the derivative optical peak value. These are the inverse square root scan rate dependence and the dependence of (dA/dE), on the n value of the system. The dependence of the derivative optical signal on the molar absorptivity of the electrode reaction product and on the concentration and diffusion coefficient of the electroactive species is to be expected. In order to establish the validity of eq 10, several experimental systems have been characterized using the DLSVA and DCVA techniques. The salient features of these systems are summarized in Table I. The dependence of the dA/dE waveform on scan rate has been established for all systems shown in Table I. Typical of these dependencies are those shown in Figure 2 for the derivative cyclic voltabsorptometric reduction of methyl viologen dication (MV2') to the corresponding cation radical (MV-) and subsequent reoxidation a t a tin oxide OTE in aqueous pH 7.00 phosphate buffer. The (dA/dE),value for the reduction of M V + is clearly dependent upon the inverse square root of the scan rate as shown in Table 11. The concentration dependence of (dA/dE), has been evaluated for each of the experimental systems listed in Table
_______--__
--__~___-----_______ Table 111. Dependence of (dA/dE), on Concentration for the Oxidation of TAAapC (dA/dE), , AU/mV X l o 5 [TAA], M x l o 5 2.52 3.21 12.6 16.7 25.5 32.9 31.6 41.9 63.2 82.5 149. 198. 255. 331. a u = 50 mV/s; C T A A + . L ) ' " T ~ A 105 M - ' s - ' ' ~at 715 Mean of nm; solution conditions given in Table I. triplicate determinations. Regression statistics for plot of (dA/dE), vs. [TAA]: slope = 1,303 (t0.008); intercoefficient of correlation = cept = 0.45 (t0.90) x 0,999 91.
'
:
I. Typical of these data are those for the oxidation of TAA in acetonitrile (Table 111) which indicate linear dependence of (dA/dE), on concentration from 2.5 X low5M to 2.5 X
M. The dependence of (dA/dE), on the molar absorptivity of the electrode reaction product and the diffusion coefficient of the precursor has been evaluated through an examination of (dA/dE), at a common scan rate for experimental systems having the same n value but widely differing ~pm~uaD1/2preclfrsor values. These are ferrocyanide oxidation in pH 2.50 glycine buffer, MV+reduction in pH 7.00 phosphate buffer, and TAA oxidation in acetonitrile. As shown in Table IV, the peak derivative optical signal is linearly dependent upon the product C ~ ~ D ' / ~ , over - - a 40-fold range of this parameter. In order to test the n value dependence of (dA/dE), predicted by eq 10, an experimental system which undergoes a
ANALYTICAL CHEMISTRY, VOL.
'' * precursor a , d
precursor
AUGUST 1981
1393
Table VI. Effect of Scan Rate on DCVA and CV Peak Potential Separationa
Table IV. Dependence of (dA/dZ)p on product1)
53, NO. 9,
(dA/dE) U/~V'
AE,(n = I), mVb -
c p r o ~ ~ ~ ~ i : p : e y o rA,
x 105 c
2.61 (iO.01) 3.23 Fe ( CN ),' MV2+ 27.5 ( t O . 1 ) 34.5 105 (+1) 129. TAA a Substrates present at 1.00 mM; u = 50 mV/s; solution Determined from single conditions given in Table I. potential step spectroelectrochemical responses. Mean Regression statistics for of triplicate determinations. plot O f (dAld1:') VS. ~DroductD1~2precursor: slope = 1.226 (+_0.007)X 10-3;)intercept= 0.35 (i0.45) X 10-5;coefficient of correlation = 0.999 98. Table V. Dependence of (dA/dE), o n Scan Rate for Oxidation of OTavC (dA/dE) u-l/z AU/rng' u , mV/s (mV/s)' x io4 2.72 0.606 10.4 7.66 4.87 0.453 8.99 0.334 5.78 15.5 0.254 4.33 27.1 0.192 3.18 48.6 0.143 2.39 90.6 0.105 1.74 a [OT] = 9.23 Y 10.' M ; C O T Z + . D ~ = /147 ~ ~ TM" s - " * Mean at 437 n m ; solution conditions given in Table I. Regression statistics for of triplicate determinations. plot of (dA/dR), vs. Y - " ~ : slope = 1.73 (tO.01) x coefficient of correlaintercept = --0.078(50.048) X tion = 0.999 83. reversible, concerted two-electron transfer at the electrode to produce an absorbing product was examined by DLSVA and DCVA. At pH 2.00, o-tolidine (OT) is known to undergo a concerted two-electron oxidation to the corresponding quinonediimine (28) (OT2'). The derivative optical response for this system as a function of scan rate is shown in Figure 3. For this n = 2 system, (dA/dE), is also inversely dependent upon the square root of the scan rate as shown in Table V. The validity of the n-value dependence predicted by eq 10 has been established by comparing the ratios of the theoretical and experimentally determined @n1/2 values for n = 1 and n = 2 systems. The dependence of (dA/dE), on C*, tD1I2,and u has been established (Tables 11-V). Therefore the ratio
,Dl/ZC*y-1 I2 affords the necessary test relationship. For the three n = 1 systems, experimental results obtained for all scan rates and concentrations examined give rise to fln1l2= -0.087 (f0.001). For the n = 2 system, @nl/' is expected to be (2)'/' times greater than this or -0.123. The experimental value of @n1/2 for n = 2, determined with the OT system for all scan rates and concentrations examined, is -0.122 (*0.001) which is in excellent agreement with the expected value of -0.123. Thus the n-value dependence of the peak derivative optical signal is as predicted by eq 10. The experimentally determined value of @, based on 195 experiments conducted on four electrochemical systems at 19 concentrations and 27 scan rates is -0.087 (f.0.002) mV-'/2 and agrees well with the theoretical
u,mV/s
DCVA
CV
2.50 2.72 4.87 4.94 8.99 15.0 15.5 25.0 27.1 48.6 50.0 90.6 97.2 150. 158. 265.
58
59
58
59
58
59
60
61
62
63
70
71
74 82
75 84
AEp(n = 2),
mV DCVA
CV
31 34
31 32
34
36
35
38
37 38
39 41
42
46
54
65
Uncertainty in ~ 1 ;measurement ~ . 2 i~ mV. [MVz+] 1.55 X 10.' M; solution conditions given in Table 1. [OT] = 1.75 X lo-" M; solution conditions given i n Table I. a
.-=
value of -0.0881 mV-1/2calculated at 25 "C. The peak potential separation observed in both DCVA and CV responses increases with scan rate. This effect is reflected in the simultaneously acquired DCVA and CV data shown in Table VI. Although the peak potentials of the DCVA and CV responses are altered due to the ohmic resistances of the OTEs (29,30),the amplitude of (dA/dE), is not perceptibly diminished over the scan ranges examined in this work as evidenced by the linear dependence of (dA/dE), on u-lI2 (Tables I1 and V). Like its optical counterpart, the normalized peak current (ip/u1/2) is, within experimental error, independent of scan rate. This invariance of the normalized current parameter indicates that the surface resistance of the OTEs, rather than double layer charging current, is the dominant contributor to the peak separations shown in Table VI. It is noteworthy that in the limit of slow scan rates, the peak separations observed in both DCVA and CV responses approach the expected values of 58 mV and 29 mV for n = 1 and n = 2, respectively (31). The limit of detection ( S I N = 2) of the linear sweep derivative optical methods is a function of the n value and tD1/2 of the system under examination, the scan rate, and the performance characteristics of the spectrophotometer employed. Taking as representative those values for the TAA system ( n = 1, C T ~ +D- 1 / 2 ~ A =A 105) and a nominal scan rate of 10 mV/s, the limit of detection has been experimentally evaluated in this present work as 1.5 x lo4 M. This limit will decrease with increasing value of n and tD1I2and decreasing scan rate. For the OT system, for example (n = 2, tm+D'/2m= 147), at the minimum scan rate employed in this work (2.50 mV/s), the limit of detection has been determined to be 3.8 x M. The dependence of the derivative optical response on scan rate makes this technique quite complementary to linear sweep and cyclic voltammetry. Depending upon the molar absorptivity of the monitored species, the derivative optical response may afford a more sensitive analytical tool than linear sweep voltammetry at the same rate of potential scan. At reduced scan rates, the analytical utility of DLSVA would be more favorable. The species selectivity attendant to the use of optical monitoring makes the derivative optical technique an attractive approach to the rapid quantitation and n-value determination of a single species of interest in systems which are not electrochemically "clean", i.e., systems where more
1394
Anal. Chem. 1981, 53, 1394-1398
than one faradaic process is ongoing. The insensitivity of the technique to nonfaradaic charge-consuming processes suggests its use in the determination of analytes in matrices such as physiological fluids which normally exhibit high background currents (32). The use of DLSVA and DCVA with multiple wavelength detection permits simultaneous multicomponent analysis with a single potential scan (32). The DCVA technique also holds considerable promise for mechanism diagnosis and kinetic characterization of homogeneous chemical reactions which are coupled to the primary electrode reaction (33). In addition, heterogeneous electron transfer kinetic parameters can also be evaluated with DLSVA and DCVA (33).
ACKNOWLEDGMENT Stimulating discussions with P. F. Seelig and the experimental assistance of E. R. Summers are gratefully acknowledged. This work was supported in part by grants from the University of Delaware Center for Catalytic Science and Technology, the IJniversity of Delaware Institute of Neuroscience (NIH Biomedical VII), and the North Atlantic Treaty Organization.
LITERATURE CITED (1) Kuwana, T.; Dariington, R. K.; Leedy, D. W. Anal. Chem. 1964, 3 6 , 2023. (2) Kuwana, T.; Winograd, N. Electroanal. Chem. 1974, 7. (3) Kuwana, T. Ber. Bunsenges. Phys. Chem. 1973, 77, 858. (4) Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1978, 9 , 241.
(5) Heinernan, W.R. Anal. Chem. 1978, 50, 390A. (6) Aibertson, D. E.; Biount, H. N.; Hawkridge, F. M. Anal. Chem. 1979,
51, 556.
(7) Bowden, E. F.; Hawkridge, F. M.; Biount, H. N. Bloelectrochem. Bloenerg. 1980, 7, 447. (8) Bowden, E. F.; Wang, M.; Bailey, J. W.; Hawkridge, F. M.; Biount, H. N. J . Nectrochem. Soc., in press. (9) Winograd, N.; Biount, H. N.; Kuwana, T. J . Phys. Chem. 196S, 73, 3456. (10) Deiahay, P.; Chariot, G.; Laitinen, H. A. Anal. Chem. 1960, 32 (6), 103A. (11) Armstrong, N. R.; Vanderborgh, N. E.; Quinn, R. K. J. Phys. Chem. 1976,80,2740. (12) Randies, J. E. B. Trans. faraday SOC. 1948, 44, 327. (13) Sevcik, A. Collect. Czech. Chem. Commun. 1948, 73, 349. (14) Reinrnuth, W. H. Anal. Chem. lS62, 34, 1446. (15) Oidham, K. B. J . Nectroanal. Chem. 1979, 705,373. (16) Nicholson, R. S.;Shain, I. Anal. Chem. 1964, 3 6 , 706. (17) Bellsteln, E I11 13, 484. (18) Weiiand, H.; Wecker, E. Ber. Dfsch. Chem. Ges. 1910, 4 3 , 699. (19) Evans. J. F.; Blount, H. N. Anal. Lett. 1974, 7, 445. (20) Shang, D. T.; Biount, H. N. J . Electroanal. Chem. 1974, 5 4 , 305. (21) von Benken, W.; Kuwana, T. Anal. Chem. 1970, 4 2 , 1114. (22) Grant, G. C.; Kuwana, T. J . Electroanal. Chem. 1970, 2 4 , 11. (23) Hawkridge, F. M.; Kuwana, T. Anal. Chem. 1973, 4 5 , 1021. (24) Strojek, J. W.; Kuwana, T. J. Nectroanal. Cbem. 1968, 76, 471. (25) Evans, J. F.; Biount, H. N. J . Phys. Chem. 1976,80, 1011. (26) Savitzky, A.; Goiay, M. J. E. Anal. Chem. 1964, 36, 1627. (27) Ito, M.; Kuwana, T. J . Electroanal. Chem. 1971, 32,415. (28) Kuwana, T.; Strojek, J. W. Dlscuss. faraday SOC. 1968, 45, 134. (29) Petek, M.;Neal, T. E.; Murray, R. W. Anal. Chem. 1971, 43, 1069. (30) Osa, T.; Yiidiz, A.; Kuwana, T. J . Am. Chem. SOC. 1969,91, 3994. (31) Matsuda, H.; Ayabe, Y. 2.Nektrochem. 1955,59, 494. (32) Biount, H. N.; Bancroft, E. E., unpublished work. (33) Biount, H. N.; Sidweii, J. S.,unpublished work.
RECEIVED for review February 17,1981. Accepted May 4,1981.
Digital Alternating Current Polarography with Microprocessor-Based Instrumentation J. E. Anderson and A. M. Bond” Division of Chemical and Physical Sciences, Deakin University, Waurn Ponds 32 17, Victoria, Australia
The technique of dlgital ac polarography is described. I n this technlque, a small amplltude digltal sine wave is applied to the cell instead of an analog sine wave. This slgnal is obtained from a microprocessor-based functlon generator and data acquisition system. Assuming that the 36-step sine wave produces a response simllar to that in conventlonal ac polarography, current data are collected every 10’ relative to the applied slgnal. By simulating the various electronic components found in conventional ac instruments, phase-sensltive detection of fundamental and second harmonics Is also posslble. A qualitative comparlson between the absolute current polarograms and of the phase-sensitive fundamental harmonlcs is in excellent agreement wlth ac polarographic theory for a reversible system.
The instrumentation used to perform ac polarography is somewhat more extensive than that used in other forms of polarography. Even in the wide variety of pulse polarographic techniques which have emerged, the only additional circuitry required (besides a basic potentiostat) is a sample and hold amplifier if a microprocessor-based function generator is used. With the exception of the more recently developed fast Fourier
transform (FFT)techniques ( I , 2) most ac instruments must also be equipped with: (1) a small amplitude ac signal source (and reference signal), (2) a high pass filter or tuned amplifier input, (3) a lock-in amplifier (phase sensitive detector), (4) phase shifting circuitry (if phase-sensitive detection is used), and finally (5) a low pass filter output (3-5). Although more and more instrument companies are providing these features as options to their polarographic instruments, they are often inflexible (with respect to ac amplitude, frequency, and phase selectivity) as well as costly. The use of on-line FFT in ac polarography is quite different from the above approach in that the only analog components which remain are the potentiostat and a low pass filter(s) ( I ) . This technique is quite elegant and a wealth of information may be obtained from a single experiment. However, a t present the necessity of a minicomputer or the equivalent in computing power still restricts the number of users of this technique. We report on here the technique of digital ac polarography which uses a microprocessor data acquisition system to mimic the analog devices mentioned above which are necessary for conventional ac polarography. As in the FFT technique, the only analog component of the instrument is the potentiostat. Unlike the FFT technique, the software required is quite simple and may be easily accomplished in assembler language.
0003-2700/81/0353-1394$01.25/00 1981 American Chemical Society
